A three-dimensional (further briefly designated as “3D”) measuring principle of focus variation for the recognition of a microscopic 3D structure of surfaces is a principle of 3D recognition using the low depth of focus of microscope optics, which has been well-known for several years. The measuring method is also described in the current ISO standard 25178-6 as a potential measuring method for the determination of high-resolution 3D geometry information in the micro- and nanometer range. With this measuring method, an image staple of an object is evaluated by means of algorithms of digital image processing, so that for each image pixel there is determined that image from the image staple, in which the respective image pixel has maximum depth of focus. The image staple is typically generated by a relative movement between object and objective of the image recording system, wherein this relative movement is typically performed along the optical axis of the objective. As this relative movement is exactly recorded via a suitable measuring system (e.g., a linear scale), a 3D model of the object may be generated by determining the most focused image pixels.
As the measuring principle of focus variation is based on the evaluation of the local image contrast, optimum sample illumination thereby is of utmost importance. In this view, classically there are two different illumination arrangements:    1. A so-called co-axial illumination, in which the light is applied onto the measuring object through the measuring objective.    2. A ring light illumination arrangement, in which the illumination elements are arranged in an annular form around the measuring objective and in which the light beams impinge laterally on the measuring object.
Both types of illumination may be used separately or also in combination. This invention relates to a type of ring light illumination that is especially suitable for the measuring principle of focus variation.
The requirements for an illumination device having a ring light for a 3D surface measuring apparatus according to the principle of focus variation are as follows:    1. As much light as possible should be directed onto that area of the measuring sample, which lies within the visual range of the measuring objective.    2. The illumination device is to have a possibly large illumination aperture, i.e. the light is to impinge on the sample across a possibly large angle range in order to ensure good illumination of relatively flat as well as steep areas of the sample.    3. The light should be as evenly distributed as possible across the illuminated angle range as well as also locally across the illuminated surface area in order to prevent disturbing highlights or artificial contrast differences. This requirement results from the fact that otherwise highlights could lead to measurement errors on weakly texturized measuring objects.    4. The illumination device or the ring light, respectively, should be efficient, i.e. as much of the electric power introduced as possible should be converted into light and directed onto the sample so that the ring light will not unnecessarily heat the measuring apparatus, thus leading to measurement inaccuracies.    5. The illumination device or the ring light, respectively, should be constructed as compact as possible in order to have a small interference contour in order to not collide with the sample during a scanning process, in which the surface topography of the sample to be examined is recognized. Furthermore, the operating distance from the measuring objective to the measuring sample should not be reduced by the illumination device or by the ring light, respectively, in order to enable recognition of an as high number of measuring samples as possible.    6. As typical surface measuring apparatuses according to the principle of focus variation have an objective revolver for the easy change of magnification, from a practical point of view it is further advantageous if the ring light may be changed quickly from one objective to the next one, thereby taking into account the different operational distances of the objectives.
The present invention addresses all these requirements and describes an illumination device that is especially well suitable for the measuring principle of focus variation.
Light sources for microscopes and surface measuring apparatuses used so far, e.g. halogen lamps, have numerous disadvantages in regard to the increasing miniaturization of components in the field of microscopy. They are expensive, require high electric power and have comparatively large dimensions, have a high heat dissipation, a short service life, and their colour spectrum is changing with the change of brightness.
From prior art there have already been known different embodiments of illumination devices, in which the light-emitting diodes (LED's) are usually arranged in an annular fashion around a measuring objective and which serve as reflected light for the illumination of microscopic specimens. From the document JP 2003-315678A there is, for example, known a ring light, which includes several light-emitting diodes for illumination, which are arranged concentrically about an optical axis of the microscope objective. The light-emitting diodes are therefore arranged, viewed in the direction of the beam passage, in a defined distance of a lens arrangement having converging lenses. The light emitted by the light-emitting diodes thereby impinges in the direction of the beam passage on a planar entry surface at a rear side of the planar-convex converging lenses, which are each convex-curved at the front side thereof or at an exit side of the light beams, respectively. The circular converging lenses thus are arranged in an annular fashion, serving to generate an as parallel as possible beam course of the light emitted by the light-emitting diodes. Further in the beam direction, the light impinges directly from the circularly arranged converging lenses on a Fresnel lens, in which the light beams entering are refracted, thereby being deflected and focused onto a focal point having a defined focal length, this is a defined distance spaced apart in the beam direction from the exit plane of the Fresnel lens. The Fresnel lens is thereby arranged according to the document JP 2003-315678A as a transmitting Fresnel lens in a way such that the staggered form thereof in the beam direction is oriented at the internal side towards the converging lenses or such that the steps of the Fresnel lens are located at the entry side of the light beams, respectively, which is why in this arrangement the transmitting Fresnel lens is used as a refraction lens. The specimen to be illuminated is disposed near or within the focal point of the Fresnel lens.
This embodiment is disadvantageous at least in so far as there will occur large losses of light during light refraction in the transmitting Fresnel lens, in particular if the refraction angle is very large. For this reason, due to refraction losses, especially light from the fringe areas of the Fresnel lens or of the ring light, will be lost, which is why the illumination intensity of the ring light in total will be reduced. The ring light, hence, meets the efficiency requirements as described above only insufficiently. Similarly, the homogeneity requirement across the illuminated angle range is met only very poorly, as the losses in the fringe area of the ring light differ greatly from the losses at the innermost ring of the ring light. Further, in the arrangement known from JP 2003-315678A there is given the disadvantage that, due to the determined distance between the light-emitting diodes and the converging lenses, stray light, which is not captured and focused by the converging lenses, will be lost. As the light-emitting diodes and, to the same extent, also the converging lenses are spaced apart from each other, there is not provided any spatially homogenous illumination of the sample, and in the illumination of the specimen using reflected light undesired highlights may appear, which will impair the image contrast of the specimens to be examined. Highlights will act like artificial contrast pixels and may lead to incorrect measurements in the evaluation of the images according to the principle of focus variation.
From the document EP 1 933 187 A1 there has been known an illumination system for chirurgical microscopes, wherein Fresnel lenses are used for focusing collinear light beams from a plurality of light-emitting diodes.
The document JP 2009258246 A discloses a prism-like Fresnel lens systems in a reflective arrangement.